An investigation on the lattice deformation of porous silicon layer was completed by means of symmetric and asymmetric X-ray double crystal diffraction. The structure change of porous layer is larger for the sample under lower current density during the anodization in hydrofluoric acid. Both the orientation offset and the lattice expansion in the direction normal as well as parallel to the surface of the matrixes have been measured. The crystal lattice of porous layer formed on the (111) silicon surface is proposed to be triangular distorted, and it gradually matches the lattice of substrate silicon after a time in the atmosphere.